Mask for x-ray lithography and method of manufacturing the same

ABSTRACT

A mask for X-ray lithography includes a transparent thin film (1) of SiC, an X-ray absorbing pattern (2) of Au formed on the surface of the transparent thin film (1) and a support member (3) of Si formed on the back surface of the transparent thin film (1). The support member (3) has an opening (4) for exposing the back surface of the transparent thin film (1). A transparent conductive thin film (5) of In 2  O 3  is formed over the back surfaces of the exposed transparent thin film (1) and the support member (3).

This application is a division of application Ser. No. 07,405,583 filedSep. 11, 1989, which is a continuation of application Ser. No.07/168,312 filed Mar. 18, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a mask for X-raylithography (hereinafter referred to as an X-ray mask) serving as anobturating member in the formation of fine patterns of not more than 1μm in size, i.e., in the submicron range, such as circuit patternsemployed for fabricating a very large scale integrated circuit device(VLSI) through a known X-ray exposure technique (X-ray lithography).

2. Description of the Prior Art

FIG. 1 is a sectional view showing a conventional X-ray mask, which isdisclosed in "X-Ray Lithography" by R. K. Watts, May 1979/Solid StateTechnology, for example. Referring to FIG. 1, a transparent thin film 1,being permeable to visible rays and X-rays, is formed by an insulator of2 to 3 μm thickness of a material which is excellent in strength, suchas BN, SiN or SiC. X-ray absorbers 2 of heavy metals having high X-rayabsorptivity, such as Au, W and Ta, are formed in lamination on thesurface of the transparent thin film 1, in correspondence to patterns tobe created. The aforementioned literature illustrates an X-ray maskwhich comprises a transparent thin film 1 of SiC and X-ray absorbers 2of a Ti film of 150 Å thickness, an Au film of 5000 Å thickness and a Vfilm of 800 Å in thickness, which are formed on the SiC thin film. Aring-shaped support member 3 is provided along the peripheral edgeportion of the back surface of the transparent thin film 1, to supportthe transparent thin film 1 and the X-ray absorbers 2. An X-raytransmission window 4, which is an opening defined in the support member3, has a region corresponding in size to the patterns to be created.

In order to perform exposure through the X-ray mask of such structure,the X-ray mask is first superposed and positioned with a substrate to beexposed, e.g., a semiconductor substrate by visible rays. Visible raysare perpendicularly applied to the surface of the X-ray mask to permeatethe same, so that the operation for superposing and positioning theX-ray mask with the substrate is performed on the basis of light visiblyreflected by the substrate. Therefore, the transparent thin film 1 mustbe permeable to visible rays as well as to x-rays. Upon completion ofsuch positioning, X-rays are perpendicularly applied to the surface ofthe substrate from above, so that those striking the X-ray absorbers 2are absorbed by the same while those striking other regions permeate thetransparent thin film 1 to enter the substrate in correspondence topatterns which are in reverse to the patterns of the X-ray absorbers 2.Thus, as noted the transparent thin film 1 must be permeable both toX-rays and to visible light.

Since X-rays thus enter the substrate in the patterns reverse to thepatterns of the X-ray absorbers 2, the quality of the patterns formed inthe substrate depends on the quality of the X-ray mask patternsTherefore, the patterns of the X-ray mask must be inspected in advanceof employment.

However, the minimum line width of patterns on an X-ray mask isgenerally less than about 0.5 μm, and hence sufficient resolution cannotbe obtained in pattern inspection through conventional opticaltechniques, whereby defects, etc., may be overlooked or not detected.Thus, pattern inspection of an X-ray mask has been performed through apattern inspection apparatus utilizing an electron beam, as disclosed,for example, in Japanese Patent Laying-Open Gazette No. 200415/1986, No.40146/1987 etc.

The conventional X-ray mask of the aforementioned structure is subjectedto pattern inspection by pattern inspection apparatus utilizing electronbeam. Thus, when an electron beam is applied to the transparent thinfilm 1 in a pattern inspection, the transparent thin film 1 iselectrified by injection of electrons, whereby the subsequently appliedelectron beam is deflected by such electrification of the transparentthin film 1. Consequently, the subsequent electron beam is deflectedfrom its target position, whereby correct pattern inspection cannot beperformed.

FIG. 2 shows another conventional X-ray mask, which has been developedto prevent such electrification of the transparent thin film 1, asdisclosed in "X-ray lithography: fabrication of masks and very largescale integrated devices", SPIE Vol. 333, Submicron Lithography (1982)and "Defect Repair Techniques for X-ray Masks∞, SPIE Vol. 471,Electron-Beam, X-Ray, and Ion-Beam Techniques for Submicron-meterLithographies III (1984). The X-ray mask as shown in FIG. 2 is differentfrom that of FIG. 1 in that a transparent conductive thin film 5 isprovided between a transparent thin film 1 and X-ray absorbers 2.According to the first of these references, for example, a Ti film of10000 Å thickness is formed as transparent conductive thin film 5 on apolyimide film of 16000 Å in thickness serving as the transparent thinfilm 1, and an Au film of 7000 Å thickness and a TaO_(x) film of 1400 Åthickness are formed thereon as the X-ray absorbers 2. According to thelatter reference, on the other hand, a polyimide film of 2 μm thicknessis formed on a BN film of 4.5 μm in thickness serving as the transparentthin film 1, and a Ta film of 300 Å thickness is formed thereon as thetransparent conductive film 5, while an Au film of 6500 Å thickness anda Ta film of 800 Å in thickness are formed as the X-ray absorbers 2. Thelatter reference mentions that electrification is caused although X-raymask patterns as created are subjected to pattern correction by focusingof an ion beam. The transparent conductive thin film 5 is permeable tovisible rays and X-rays, similarly to the transparent thin film 1. Inthis X-ray mask, electrons charged in the transparent thin film 1 escapeto the ground through the transparent conductive thin film 5.

In case of thus sandwiching the transparent conductive thin film 5between the transparent thin film 1 and the X-ray absorbers 2, materialsand film forming conditions must be determined with consideration ofdifferences in their adhesive properties and the differences inexpansibility between the transparent thin film 1, the X-ray absorbers 2and the transparent conductive thin film 5 in order to manufacture theX-ray mask. The manufacturing steps in such a process are complicated,with a requirement for strict manufacturing conditions.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been proposed to solve theaforementioned problem, and an object thereof is to provide an X-raymask which can be easily manufactured to relax electrification of atransparent thin film as well as to obtain sharp patterns in X-rayexposures therethrough.

The X-ray mask according to the present invention comprises atransparent thin film that is permeable to at least visible rays andX-rays and X-ray absorbing layers formed on the major front surface ofthe transparent thin film in a selectively spaced manner to absorb atleast X-rays. A support member is formed on the back surface of thetransparent thin film to support the transparent thin film and the X-rayabsorbing layers. This support member has an opening for exposing atleast the back surface of the transparent thin film. Furthermore, aconductive thin film of a conductive substance that is permeable to atleast visible rays and X-rays is formed over the back surfaces of theexposed transparent thin film and the support member.

A method of manufacturing an X-ray mask according to the presentinvention comprises the steps of:

(i) preparing a support substrate;

(ii) forming a transparent thin film that is permeable to at leastvisible rays and X-rays on the major front surface of the supportsubstrate;

(iii) selectively removing the back surface of the support substrate todefine an opening for exposing at least the back surface of thetransparent thin film;

(iv) forming an X-ray absorbing layer, for absorbing at least X-rays, onthe major front surface of the transparent thin film;

(v) forming a conductive thin film of a conductive substance that ispermeable to at least visible rays and X-rays over the back surfaces ofthe exposed transparent thin film and the support substrate; and

(vi) selectively removing parts of the X-ray absorbing layer to formX-ray absorbing patterns in a spaced manner.

In a preferred embodiment of the present invention, the conductive thinfilm includes an In₂ O₃ film. The support member is provided in the formof a ring, while the conductive thin film is grounded.

According to the inventive X-ray mask, electrons charged in thetransparent thin film in pattern inspection by incidence thereon of anelectron beam, etc., are emitted to the ground through the conductivethin film. Since the conductive thin film is formed over the backsurfaces of both the transparent thin film and the support member, thetotal thickness of the transparent thin film and the conductive thinfilm is uniform regardless of the positions of X-ray application. Thus,sharp patterns can be obtained to be transferred in the X-ray exposurethrough the mask.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a conventional X-ray mask;

FIG. 2 is a sectional view showing another conventional X-ray mask;

FIG. 3 is a sectional view showing an X-ray mask according to anembodiment of the present invention;

FIG. 4 illustrates the principle of relaxing electrification of atransparent thin film in the X-ray mask according to the presentinvention;

FIG. 5 is a sectional view showing a reference example of an X-ray mask;and

FIGS. 6A, 6B, 6C, 6D, 6E and 6F are sectional views illustrating varioussteps in a method of manufacturing the X-ray mask according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a sectional view showing an embodiment of the presentinvention. Referring to FIG. 3, an X-ray mask according to the presentinvention is absolutely identical to the conventional X-ray mask asshown in FIG. 1, except for that a transparent conductive thin film 5,preferably formed of In₂ O₃ ; is formed in lamination over the backsurfaces of a transparent thin film 1 and a support member 3. Thetransparent conductive thin film 5 is formed of a material selected tobe permeable both to visible rays and to x-rays, similarly to film 5shown in FIG. 2 on the upper surface of transparent thin film 1.

With reference to FIG. 4, description is now provided on howelectrification of the transparent thin film 1 is relaxed in the caseof, for example, a pattern inspection of the X-ray mask through exposureto an electron beam, as illustrated in FIG. 3. When an electron beam isperpendicularly applied to the surface of the X-ray mask, thetransparent thin film 1 is electrified in regions not covered by X-rayabsorbers. However, the transparent thin film 1 is merely 2 to 3 μm inthickness, and hence charged electrons easily move toward thetransparent conductive thin film 5. Since the transparent conductivethin film 5 is grounded to the exterior, such electrons pass through thetransparent conductive thin film 5 to be emitted toward the exterior ofthe X-ray mask. Thus, electrification of the transparent thin film 1 canbe relaxed.

While the transparent conductive thin film 5 is formed over the backsurfaces of the transparent thin film 1 and the support member 3 in theaforementioned preferred embodiment of this invention, an examplecomprising a transparent conductive thin film 5 formed on the surfacesof a transparent thin film 1 and X-ray absorbers 2 is illustrated forreference purposes. FIG. 5 is a sectional view showing such a referenceexample with respect to the above embodiment. This X-ray mask isconsidered to be relatively easily manufactured to preventelectrification of the transparent thin film 1. Referring to FIG. 5, theX-ray mask illustrated therein is different from the prior art as shownin FIG. 1 in that the transparent conductive thin film 5 is uniformlyformed in lamination over both the transparent thin film 1 and the X-rayabsorbers 2. Similarly to that shown in FIG. 2, the transparentconductive thin film 5 is permeable to visible rays and X-rays, whilebeing grounded to relax electrification of the transparent thin film 1.

The X-ray mask having the structure illustrated in FIG. 3 can berelatively easily manufactured as compared with that shown in FIG. 2.However, assuming that X-rays are perpendicularly applied to the surfaceof the X-ray mask from above, a difference in X-ray intensity is causedbetween X-rays passing through portions of the transparent conductivethin film 5 and the transparent thin film 1 close to edge portions ofthe X-ray absorbers 2 and those X-rays passing through other portions ofthe transparent conductive thin film 5 and the transparent thin film 1.Consequently, the patterns transferred in X-ray exposure through themask are not sharply defined.

Description is now provided on a method of manufacturing the X-ray maskas shown in FIG. 3. FIGS. 6A to 6F are sectional views illustratingsteps in a preferred embodiment of the method of manufacturing the X-raymask according to the present invention.

Referring to FIG. 6A, a transparent thin film 1 of BN, SiN, SiC or thelike, having a in thickness of 2 to 3 μm, is uniformly formed inlamination on a support substrate 13 of Si or the like that is beingformed to thickness of about 1 to 2 mm by low pressure chemical vapordeposition (LPCVD). Then, as shown in FIG. 6B, wet etching is performedon the back surface of the support substrate 13 through photolithographyto define a region for transmitting X-rays in correspondence to asubstrate, thereby to form a support member 3 and an X-ray transmissionwindow 4. Referring to FIG. 6C, an X-ray absorber 2 of a material havinghigh X-ray absorptivity such as Au, W, Ta or Pb is uniformly formed inlamination on the surface of the transparent thin film 1 to be inthickness of 0.8 to 1 μm, by sputtering. Then, as shown in FIG. 6D, atransparent conductive thin film 5 of In₂ O₃ of about 1000 Å inthickness is uniformly formed in lamination over the back surfaces ofthe transparent thin film 1 and the support member 3. The transparentconductive thin film 5 can be formed by any known film forming methodsuch as sputtering, chemical vapor deposition (CVD), etc. Referring toFIG. 6E, mask patterns 6 of resist corresponding to desired circuitpatterns or the like are formed on the X-ray absorber 2 through anelectron beam exposure technique (electron beam lithography). Then, asshown in FIG. 6F, dry etching is performed on the X-ray absorber 2through the mask patterns 6, to leave only regions of the X-ray absorber2 corresponding to the mask patterns 6 while removing other regions.Thereafter the mask patterns 6 are removed, to define desired patternsof the X-ray absorber 2 on the transparent thin film 1.

Through the aforementioned steps, an X-ray mask capable of relaxingelectrification of the transparent thin film 1 can be manufactured.According to the aforementioned manufacturing method, only one surfaceof the transparent conductive thin film 5 relates to junctionparticularly in the step (FIG. 6D) of forming the transparent conductivethin film 5 in lamination over the back surfaces of the transparent thinfilm 1 and the support member 3, whereby manufacturing conditions suchas the material for the transparent conductive thin film 5 and filmforming conditions can be relatively freely determined as compared withthe known X-ray mask, illustrated in FIG. 2 having the transparentconductive thin film 5 in a sandwich structure. In the mask manufacturedaccording to the present invention, in a region of the transparentconductive thin film 5 corresponding to the X-ray transmission window 4,the transparent thin film 1 and the transparent conductive thin film 5are substantially uniform in thickness with respect to the direction ofincidence of X-rays, hence X-rays permeating the transparent thin film 1and the transparent conductive thin film 5 to reach the substrate aresubstantially equal in intensity regardless of the position of theirapplication, which eliminates problems of poor contrast due todifferences in X-ray intensity between edge portions of the X-rayabsorbers 2 and other portions as shown in FIG. 5.

Also, in pattern correction by ion beam or pattern etching by ionplasma, electrification of the X-ray mask can be relaxed by forming theX-ray mask similarly to the above embodiment.

Although the transparent conductive thin film 5 is formed of In₂ O₃ inthe above embodiment, the same may be formed of ZnO. Alternatively, thetransparent conductive thin film 5 may be formed by a metal thin film ina thickness permeable to visible rays. However, if such a metal thinfilm is prepared by a material having high X-ray absorptivity, thethroughput in any X-ray exposure is reduced. Therefore, the metal thinfilm is preferably formed of a light metal such as Ti.

Further, although the transparent conductive thin film 5 is formed inlamination over the back surfaces of the transparent thin film 1 and thesupport member 3 after the X-ray absorber 2 is formed in uniformlamination on the surface of the transparent thin film 1 (FIG. 6C) inthe aforementioned steps of manufacturing the X-ray mask, thetransparent conductive thin film 5 may be formed in any stage so long asit is formed after formation of the support member 3 and the X-raytransmission window 4 and before etching of the X-ray absorber 2.

According to the present invention as hereinabove described, thetransparent conductive thin film is formed in lamination over the backsurfaces of the transparent thin film and the support member, wherebyelectrification of the transparent thin film is relaxed in patterninspection of the X-ray mask by electron beam etc. Thus, an X-ray mask,which enables pattern inspection, etc., to a higher accuracy, can berelatively easily obtained. Through employment of such an X-ray mask,sharp patterns can be created in X-ray exposure.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of inspecting by an electron beam a pattern of an x-ray lithography mask having an x-ray absorbing layer patterned on a front surface of a transparent thin film, comprising the steps of:forming a conductive thin film on a rear surface of said transparent thin film to provide a path for electrons charged in said transparent thin film; applying an electron beam to the mask at an outside surface of the x-ray absorbing layer to inspect a pattern thereof; and connecting said conductive thin film to ground.
 2. The method of claim 1, wherein said conductive thin film is formed from the group of materials consisting of In₂ O₃, ZnO and Ti.
 3. The method of claim 1, wherein said x-ray absorbing layer is patterned by etching. 